The past decade has seen a dramatic increase in demand for freshwater foodfish, such as catfish, as well as many varieties of baitfish. As the aquaculture industry expands and matures, more efficient farming methods must be developed. One problem that has plagued aquaculturists is the difficulty of predicting harvest dates and maintaining accurate inventory records. This problem is confounded by stocking fish of various sizes and also due to natural processes such as competition for food, which results in a pond with fish of various sizes.
The aquaculturist must be able to predict the most probable harvest date at which an adequate number of fish in a particular pond have reached the optimum harvest size. The problem of size variance in a fish pond makes prediction of the best harvest time difficult. Computer software applications such as Fishy 3.2 (developed at Mississippi State University) can significantly improve harvest predictions, provided that population size distribution information is available at stocking. A simple and efficient method of sampling a fish population to determine the relative size distribution of fish being stocked in a particular pond would greatly increase the accuracy of a farmer's best harvest date prediction, and thereby maximize a farmer's profits when harvest is performed as closely to the ideal date as possible.
Accurate estimation of harvestable fish inventories is important for fish processing facilities as well. Harvested fish are transported live to the processing facility to ensure a high-quality product. For maximum efficiency, processors must schedule the arrival of fish at the facility so that the processing equipment is continuously in operation but also so that a minimum number of fish are waiting to be processed. If a farmer's earlier estimate of the harvest date is wrong, such that the number of fish harvested is more or less than expected, the scheduling necessary for efficient processing is made impossible. The result is inefficient use of fish processing capital and possible spoilage or additional handling of harvested fish.
Another problem within the aquaculture industry relates to the valuation of fish at the time of harvest. For transport to the processing facility, harvestable size fish are typically removed from the growing pond and placed in a tanker truck. Unlike many other farm products, it is not possible to accurately value fish based merely on the raw weight of fish collected. This is because fish in different size categories have different per pound values. For example, catfish processors utilize automatic fillet machines that are calibrated for fish in a particular size range. Fish that are too large or two small must be processed by hand and are therefore worth less on a per pound basis. A standardized method is needed to sample the size distribution of a population of fish at harvest time that is quick enough to not significantly slow the progress of the fish to the processing facility. A significant delay at this critical time can result in additional stress to the fish and potential spoilage before the fish arrive at the processing facility.
Numerous fish grading mechanisms have been developed to solve one or more of these problems. One type of prior art fish grader consists of a series of gratings in a floating box through which fish are poured. The largest fish stop on the largest grating, whereas smaller fish pass through and stop on one of the gratings with progressively narrower openings. One problem with this fish grader is that the fish must align themselves with the grating bars and must physically swim through the grating bars if they are small enough. Fish that are disoriented, which is often the case during handling, tend to be caught on the larger grating even if they might pass through if aligned properly. This type of grader is particularly inappropriate with catfish, since catfish typically extend their fins when under stress and thereby hold themselves between the bars of a grating large enough for them to otherwise slide through. Such fish will thus be misgraded as a size larger than they actually are.
Another problem with this type of grader is that since there is little or no water flow to the fish during the grading process, the fish may suffer health problems from oxygen depletion. This problem is particularly acute because fish are typically quite crowded when entering the main bin of a fish grader before the grading process begins. The high fish density and lack of water flow may also lead to other problems related to water quality.
This type of fish grader also suffers from a problem related to the design of the gratings themselves. To accurately grade fish, the bars in the gratings must be maintained at a precise distance from each other along their entire length. Prior art gratings accomplish this by means of a cross bar attached to the grating bars at various points along their length. A fish's tail may easily hang on these crossbars, thereby preventing the fish from slipping through the larger grating, resulting in a fish that is graded as a size larger than its actual size.
Yet another problem with this type of fish grader is that once fish are caught on the grader panel, the panel must be turned over so that the fish are removed. This process places stress on the fish both because the fish are removed from the water and because the fish are jostled about as the grader panel is turned over. This process is also labor intensive and not practical for large numbers of fish.
Another type of prior art fish grader consists of a box with progressively smaller gratings placed horizontally with respect to each other. Water is forced through the box from the end with the smallest grating to the other end with the largest gratings, and fish swim against the current from the largest grating end to the point where they encounter a grating through which they can no longer pass. A significant problem with this type of grader is that fish tend to swim back between the bars of the larger gratings before the distinct size groups can be removed, and are thereby misgraded. Also, removal of the various size groups is accomplished through manually dipping the fish out, which is not practical as part of a large-scale aquaculture operation.
A third type of prior art box-type grader, described in U.S. Pat. No. 4,198,925 issued to Lindbergh, consists of a floating box which is generally open to the top with at least one side having a grating through which fish of a small enough size can pass through. The major problem with this type of grader is that it can only grade one size group at a time and relies on passive grading. Fish must be crowded to encourage grading.
The aforementioned graders are relatively slow in operation. The speed of the grading operation is particularly important when fish are being graded as the fish are being loaded for transport to a processing facility. Because of the extremely short time during which fish must be processed after removal from the water, any delay during the collection process may result in down time at the processing facility and/or additional stress on the fish.
A fourth type of prior art fish grader operates by inserting one fish at a time into an opening at the junction of two conveyor belts set at an angle to one another. One example of such a device is the Fischtechnik Model FGM 12/1, produced by Fischtechnik Fredelsloh of Moringen, Germany. At the near end, the belts of this device form a V-shaped channel in which a fish is placed. The conveyor belts are farther apart at their far end, resulting in a gradually increasing gap between the belts as the fish is pulled by the conveyor belts toward the far end. As the fish travels along the belts, the gap eventually becomes large enough that the fish drops through to one of several waiting collection bins below. The size of the fish determines how far along the belts the fish is when it drops, and therefore which bin into which the fish falls. This type of grader is extremely slow in operation, since it can only handle one fish at a time. It is also inaccurate since fish can easily catch their fins on the conveyor belts as they travel and thus fall into a bin designed to catch larger fish. This problem is particularly acute with catfish, which tend to extend their fins when under stress such as that caused by their removal from the water. This type of fish grader is also quite expensive to build because of the large number of precision moving parts.
A fifth type of prior art grader, described in U.S. Pat. No. 4,351,438 issued to Morton, consists of an apparatus that includes an elevated tank. Fish travel from this tank by gravity through a series of gratings that separate the fish into different size groups. One problem of this grader is that the fish may strike the gratings perpendicular to the grating bars and be misgraded as a result. Also, fish species such as catfish may extend their fins and thereby inhibit grading when the fish is out of the water.